Identifying genes and gene products that are important in tumor cells is a high priority goal of the National Cancer Institute. In order to determine whether a gene of interest is expressed in human malignancy, investigators must be able to evaluate actual tissue samples of human tumors. Unfortunately, access to large numbers of well-characterized human tissue samples is difficult, and there is a large expense for the preparation of glass slides from tumor in order to perform immunohistochemistry or in-situ hybridization. As a partial solution to this problem, pathologists began to combine multiple tissue samples in one tissue block, in order to reduce the histology costs and the time and effort involved to perform the special studies. This technique has been refined to the point where hundreds of tissue samples can be placed in a grid arrangement in a single paraffin tissue block. Investigators in the NHGRI (Kohenen, Kallioniemi and others) demonstrated the utility of large scale tissue microarrays in a seminal paper published in Science. Following a tissue microarray workshop hosted by NCI in Fall, 1999, a steering committee convened with the purpose of establishing a pathology-based tissue microarray core facility. The steering committee is composed of representatives from intramural and extramural NCI and from the NHGRI. This project is a direct outgrowth of the Extraordinary Opportunity to Define the Signatures of Cancer Cells identified in the 2001 Bypass Budget plan. The goals of the first year were to establish a core microarray production facility based on a similar facility in the NHGRI and to create the first mixed tumor microarrays for nationwide distribution. The initial product of the core was a mixed tumor block containing representative samples of the most common epithelial malignancies (breast, colon, lung, prostate, and ovary) as well as samples of melanoma, glioma and lymphoma. A selection of normal tissue and standard cell lines was also included to bring the total number of tissue spots to 500. Anonymized human tumor samples are obtained through the CHTN, and this organization will also handle distribution of arrays on glass slides to investigators in the intramural and extramural scientific community. Four sets of multi-tumor arrays were prepared in the first production year (FY2001), totaling more than 3000 slides for distribution. This past year (FY2004) we have built arrays of plasmablastic lymphomas, head and neck carcinoma, breast carcinoma (for the Polish Breast Cancer project), and an array of pediatric tumor xenografts. We will also continue to produce a diverse multitumor array for general distribution. Array projects for the coming year include continuing work on an array of the NCI60 cell lines with DTP and other epidemiology cancer collection with DCEG. In addition to slide production, the core facility has been engaged in technology development, from the basic histology procedures to array use to large-scale imaging of arrays. We have acquired an automated arrayer and have worked collaboratively on further technology development of the automated arrayer and an automated imaging system. In addition to the tumor arrays for the CHTN, we produce smaller custom arrays for NCI and NIH investigators on a collaborative basis.